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Cortical processing of information is not restricted to the striate cortex. About thirty
other cortical areas have been identified. These cortical areas can be differentiated
mainly by the selectivity properties of their neurons.
3.2.1.2 The eye
The function of the eyes is to channel the light of a wavelength in the range of 400
to 700 nm, emitted or reflected by an object to create a clear image which is printed
on that part of the eye which is covered by sensory receptors, i.e. the retina. The eye
is composed of a series of reflective media that play the role of a convex lens whose
total focal distance can vary with a modification of the curve of the crystalline lens
(Figure 3.3).
In broad sense, the eye is a spherical darkroom. There are several dioptres at its
entrance and the receptor structure at the rear:
The pupil is the diaphragm of the system. The light rays are then projected on the
retina which serves as a spherical screen;
The cornea is a fibrous, transparent membrane which constitutes the main lens
of the optic system. It is the anterior pole of the eye and the optical axis passes
through its centre. Its horizontal diameter is 11.5 to 12mm;
The crystalline lens is a biconvex transparent lens placed between the iris and the
vitreous body.
3.2.1.3 Accommodation and convergence
While looking at an object at less than approximately 65 meters, the reflected image
would become blurred on the retina if the eye did not have the ability to accommodate
automatically. As a result of the action of the ciliary muscles, the power of the crys-
talline lens varies and makes it possible to focus on the retina to see objects that are
near or at a distance. (Figure 3.4): This phenomenon is called accommodation. The
crystalline lens is the only dioptre of the visual chain whose power is variable. All light
rays are diverted towards the fovea, which is the central point of the retina. The level
of accommodation is adjusted to obtain a clear image.
The muscles of the orbital globes make it possible to orient the two eyes by
converging them to the point in the space observed. This phenomenon is called
convergence.
Figure 3.3 Anatomic structure of the human eye
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